This invention relates to an electron beam apparatus such as an electron microscope and, in particular, to an improved gas exhaust system for use in an anode chamber for an electric field releasing type electron gun which requires a high vacuum level.
In general, the performance of an electron beam apparatus, such as an electron microscope, is greatly influenced by the brightness of the electron gun (corresponding to an emitting current density for a unit solid angle).
A thermal electron emitting type electron gun has been reduced to practice which uses lanthanum hexaborite as a cathode material for obtaining a high brightness level. Consequently, the field emission type electron gun has a wide range of applications. Tungsten and titanium carbide are primarily used as the cathode material of the field emission type electron gun. In order for the electron gun to operate stably, it is necessary that a high vacuum level, of the order of 10.sup.-9 to 10.sup.-11 Torrs, be obtained.
Generally, as shown in FIG. 1, ion pump 11 is coupled to gun chamber 13 of electron gun 15, via exhaust tube 12. The exhausting of a gas within gun chamber 13 is carried out by a residual gas trap section (i.e., exhaust zone) within ion pump 11.
FIG. 2 shows one form of gas exhaust section 14 incorporated within ion pump 11. In FIG. 2, 21 shows cylindrical electrodes biased to a positive potential; 22 shows a parallel magnetic field; and 231 and 232 show plate-like electrodes biased to a negative potential, which function as getters. In the ion pump, magnetic field 22 is applied in a direction parallel to the axis of cylindrical electrodes 21 and electrons within cylindrical electrodes 21, accelerated by the electric field generated among electrodes 21 and 231, 232, are moved in spiral fashion.
In the current technique, a vacuum level attained by the ion pump per se is of the order of 10.sup.-10 Torrs. In the arrangement shown in FIG. 1, the large distance between electron beam emitting section 10 and gas exhaust section 14 within ion pump 11, arises due to the mechanical geometry of ion pump 11, exhaust tube 12, and anode chamber 13. Even if a vacuum level of the order of 10.sup.-11 Torrs is obtained at the location of the ion pump, the vacuum level of anode chamber 13 drops to about 10.sup.-9 to 10.sup.-10 Torrs. Thus, in the conventional apparatus, as shown in FIG. 1, the vacuum level within the anode chamber is reduced by one to two orders of magnitude in comparison with that obtained at the ion pump.
In order to eliminate the aforementioned drawbacks, two solutions can be considered: (1) ion pump 11 is located as near to gun chamber 13 as possible, and (2) the exhausting capacity of ion pump 11 is made as large as possible. In the case of solution (1), there is a limit as to how close ion pump 11 can be located vis-a-vis chamber 13, due to exhaust tube 12 and flange 120 being interposed therebetween. With respect to solution (2), other problems arise, such as the price, size, and weight of various large-capacity ion pumps.